Dane Paterson struck first, nipping one back to bowl Luke Wells before Jennings and Josh Bohannon steered Lancashire to 59-1 in a slightly abbreviated morning session but the visitors fought back strongly with three wickets in the afternoon.
Wicketkeeper Joe Clarke dropped an edge by Bohannon (25) off Olly Stone but redeemed himself by holding onto a near-identical chance off the very next ball.
Pennington had George Bell caught at second slip after an uncomfortable 39-ball stay and Matty Hurst tamely chipped Lyndon James to Stone at midwicket to leave Lancashire 120-4.
Jennings held the innings together with a mixture of calculated attack and strong defence, the left-hander depositing Paterson over square leg for six while his 16th four brought up a 179-ball hundred.
George Balderson made 41 and lofted consecutive Liam Patterson-White deliveries into the Harrod Drive gardens overlooking the ground in an 88-run partnership.
In a lengthy 41-over 194-run final session, Chris Green and Tom Bailey (20) were both lbw to James (3-53) while Will Williams edged Pennington behind for a duck.
Jennings opened up as he started to run out of partners, taking 47 balls to go from his hundred to 150 as he deposited Stone for six onto the adjacent railway line and ramped Paterson over long-leg for six more.
The captain was dropped three times during the evening as Lancashire built a strong total that they will look to bolster on Monday, when Anderson is likely to both bat and bowl.
Report supplied by ECB Reporters’ Network
